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Dye color of MIL-H-5606 hydraulic fluid

Aviation Mechanic Airframe (AMA) practice question 6 of 15 on Hydraulic and Pneumatic Systems. ACS element AM.II.F.K1 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONMineral-based hydraulic fluid (MIL-H-5606) is dyed what color?

  1. ABlue
  2. BPurple
  3. CRed
Show the answer and explanation

Correct answer: C. Red

  • A — incorrect Blue is not used as a fluid dye; blue appears in the color coding of O-rings that are compatible with MIL-H-5606, not in the fluid itself.
  • B — incorrect Purple is used for some synthetic hydraulic fluids, not for the petroleum-based mineral fluid.
  • C — correct Mineral-based hydraulic fluid processed from petroleum has an odor similar to penetrating oil and is dyed red.

BACKGROUND

Hydraulic fluids fall into three principal categories: minerals, polyalphaolefins, and phosphate esters. Mineral oil-based fluid, MIL-H-5606, is the oldest of these, dating back to the 1940s, and is processed from petroleum. It is used in many systems, especially where the fire hazard is comparatively low, and it can be recognized by an odor similar to penetrating oil and by its red dye. Some synthetic hydraulic fluids are dyed purple and even green, depending on the identity of the fluid. Synthetic rubber seals are used with petroleum-based fluids.

STUDY TIP

Learn the three fluid categories together with the military specification numbers and the identifying dye, because several exam items hinge on recognizing a fluid by specification rather than by appearance alone.

KEY POINTS

  • MIL-H-5606 is a mineral (petroleum) based hydraulic fluid dyed red.
  • Some synthetic fluids are dyed purple or green.
  • Always confirm fluid type from the maintenance manual or the reservoir instruction plate, not from color alone.

SOURCES

  • sources/ama-airframe/ch12.txt "Mineral-Based Fluids"
  • sources/ama-airframe/ch12.txt "Types of Hydraulic Fluids"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.